This invention relates to a transfer switch of the type that is adapted to be interconnected with an auxiliary power source, such as an electrical generator, for controlling the supply of electrical power from the generator to branch electrical circuits in an electrical load center. More particularly, the invention relates to a transfer switch that connects the neutrals of a load center in a single and switchable connection thereby allowing connection and disconnection of the load center neutrals to a primary power source or an auxiliary power source in a single operation.
A building, such as a home or other dwelling, will present critical and non-critical loads to the primary power source of the building, which is generally a utility power supply. The critical loads for a home, for instance, may include the HVAC system, sump pump, refrigerators, freezers, dishwasher, washer/dryer, and life-sustaining medical equipment. All other loads of the home will generally be considered non-critical. The non-critical loads are generally connected to non-critical branches that are hardwired to a load center and the critical loads may be connected to critical branches that are hardwired to a separate subpanel; both of which are powered by the primary power source during normal primary power source operation.
To ensure power to the critical loads during primary power source failure, it is known to connect the subpanel and, thus, the critical loads, to an auxiliary power source, such as electrical generator using a transfer switch. The majority of prior art transfer switches are manually operated. With transfer switches of this type, the operator initiates operation of an auxiliary power source, such as an electric generator, and connects the auxiliary power source to the transfer switch, unless there is a permanent connection between the generator and the transfer switch. The individual switches or circuit breakers of the transfer switch are then actuated to supply power from the generator to the circuits in which the individual transfer switches are connected.
As shown in FIG. 1, a conventional manual transfer switch 10 is operative to selectively connect a series of loads 12 between a primary power source 14 and an auxiliary power source 16. In the illustrated example, the manual transfer switch 10 is a four-circuit, load-side, non-separately-derived transfer switch. In this regard, the manual transfer switch 10 services four loads, represented by resistors R1, R2, R3, and R4 and performs any switching of the loads 12 between the primary and auxiliary power sources 14, 16 using hot switches 18 after utility breakers 20 and distribution breakers 22. As evident in the schematic, the primary power source 14 includes two hot lines 24, 26 and a neutral line 28 and the auxiliary power source 16 similarly includes two hot lines 30, 32 and a neutral line 34. As further illustrated in the schematic, the neutral lines 28, 34 are permanently connected to, rather than switched between, the primary power source 14 and the auxiliary power source 16. Because of this permanent connection, the manual transfer switch 10 could not be used in a separately-derived, load-side manner, as may be desired.
Because of this desirability to have a separately-derived configuration, one proposed solution to the drawbacks presented by a non-separately-derived, load-side transfer switch, such as that shown in FIG. 1, is a transfer switch in which a neutral switch is used for each hot switch, as shown in FIG. 2. In this example, transfer switch 10 includes two hot switches 18 and two neutral switches 36 to selectively connect neutral wires 28, 34 of the primary power and auxiliary power sources 14, 16, respectively, to loads 12.
In the proposed solution shown in FIG. 2, the number of hot switches 18 is effectively reduced by one-half to account for the neutral switches 36. That is, if the transfer switch 10 is configured to have four switches, two of those switches are used as hot switches 18 and the remaining two switches are used as neutral switches 36. Alternately, the transfer switch 10 may be constructed to have a total of eight switches to provide the same number of hot switches 18 provided by the transfer switch 10 illustrated in FIG. 1. Doubling the number of switches effectively increases the size, wiring complexity, and cost of the transfer switch.